This invention relates to additive compositions, otherwise known as admixtures, for incorporation in hydraulic cement mixes, for example, hyraulic cement concretes, mortars, and grouts, neat cement mixes, concrete block mixes, and dry mixes for making such concretes, mortars, grouts, especially to accelerate their rate of hardening and setting.
A variety of techniques have been employed to accelerate the hardening of hydraulic cement mixes. These techniques are employed because of circumstances or conditions that render unduly extended the time required for hardening of such mixes in given applications. The rate of hydration of portland cement is very dependent upon temperature, for example, so that concrete containing it will often harden at a slower rate than desired during the winter season unless provisions are taken to accelerate the hardening process. Among the various techniques employed for this purpose are the increasing of the proportion of portland cement in the mix; the use of more rapid setting types of cements available; the heating of the water and other components of the concrete; and the use of chemical admixtures that act, catalytically or otherwise, to increase the rate at which the concrete hardens.
A number of chemical agents that serve to accelerate the rate of hardening of concrete are known in the art. Calcium chloride in particular is well known as an effective and economic accelerator. In use, however, calcium chloride is known to have certain disadvantages, principally at elevated usage rates it tends to promote corrosion of metal embedded in, or in contact with, the calcium chloride-containing concrete. Other agents which do not promote corrosion of metal, but have a less pronounced effect in accelerating the rate of hardening of concrete include: alkanolamines, aldehyde condensates of urea and melamine disclosed in U.S. Pat. No. 3,785,839 issued to Peppler, et al., on Jan. 15, 1974, U.S. Pat. No. 3,864,290 issued to Peppler, et al., on Feb. 4, 1974, and U.S. Pat. No. 3,767,436 issued to Peppler, et al., on Oct. 23, 1973; formate salts disclosed in U.S. Pat. No. 4,261,755 issued to Berry, et al., on Apr. 14, 1981 and U.S. Pat. No. 4,419,138 issued to Popovics on Dec. 6, 1983; thiocyanates disclosed in U.S. Pat. No. 4,373,956 issued to Rosskopf on Feb. 15, 1983; carbonates and carbonyl compounds disclosed in U.S. Pat. No. 4,264,367 issued to Schutz on Apr. 28, 1981 and U.S. Pat. No. 4,257,814 issued to Kellet, et al., on Mar. 24, 1981; and alkali-metal nitrates, thiocyanates and alkanolamines disclosed in U.S. Pat. No. 4,473,405 issued to Gerber on Sept. 25, 1984.
There is a continuing need in the art, therefore, for improved set accelerating agents. In particular, there is a need for stable and safe set accelerating agents capable of rapidly accelerating the rate of set of portland cement mixes. In addition, of course, there is a continuing desire in the art for admixtures capable of permitting other advantages, such as reduction of the water content of the mix and improved compressive strength of the hardened concrete.
Another use for chemical accelerators is to overcome retardation caused by strength enhancing admixtures. Many admixtures employed to achieve improved compressive strength are known to act also as set retarders, and such admixtures slow the chemical process of hydration so that the concrete remains plastic and workable for a longer time than concrete without such a retarder. While admixtures having set retarding and compressive strength improving properties are useful per se, frequently there are instances where improved compressive strength is desired by any significant retarding of the rate of hardening of the cement or concrete mix would be undesirable. In such an instance, it is desirable to overcome the retarding effect by using accelerators.